Production of nitrocellulose



@MyL W June 13, 1933. M UR 1,913,416

PRODUCTION OF NITROCELLULOSE Filed May 13, 1930 35 high alpha cellulose content and was Patented June 13, 1933 UNITED STATES PATENT OFFICE I MILTON O. SGHUB, OF' BERLIN, NEW EAMPSHIRR-ASSIQNOR '10 BROWN COMPANY, OF

BERLIN, NEW HAMPSHIRE, A CORPORATION OF MAINE PRODUCTION NITBOOELLULOSE Application filed Kay 13,

This invention is directed to the production of nitrocelluloses of all varieties, such as used, for example, as raw material in the manufacture of plastics, lacquers, films, artificial silk,

explosives, and kindred products.

5 and so on. It is also known that the physical form of the cellulose during nitration is an important factor in determining the course of the. reaction.

After considerable investigation of nitration problems, I have discovered that marked advantages may be realized by carrying out the nitration of cellulose in two or more stages, the first-stage nitration being elfected with an' acid rich in nitric acid, and the second-stage nitration withanacid less rich in nitric acid. While not restricted thereto, such a discovery may be applied most profitably when the cellulose is in the form of comparatively thick sheets of unbeaten wood pulp, such as the so-called drier sheets available on the market for viscose-rayon manufacture. Heretofore the nitration of such sheets was attended by manyserious obstacles, even when the pulp was refined to reasonably expected to give good results. Thus, if such sheets are al owed to undergo nitration to completion in the usual. nitrating acid at normal temperatures, hydrolysis takes'place to such an extent that the yield is inordinately low; the sheets tend to disintegrate;

the acid retained by the nitrocellulose is excessively high; and the nitrocellulose is comparatively non-uniform and otherwise lack mg in optimum characteristics. By apply- 1930. Serial .No. 451,995.

ing the principles of the present invention, however, drier sheets as thick as 0.040 inches or even greater, may be satisfactorily nitrated intwo to four hours at temperatures as high as or 40 0., or even higher. 4

The use of unbeaten wood pulp in the form of thick sheets for the direct productionof nitrocellulose constitutes an economic advantage of the first magnitude, as it does lieved necessary for best nitration results.

The production of thick sheets of unbeaten wood pulp, say, 0.020to 0.050 inches thick is carried out at comparatively low cost at a pulp mill where it is customary practice to pump the finished pulp to so-called pulp driers, which are built as large units capable of handling at low cost 150 to 200 tons or more of fiber per day. Again, the washing, stabilizing,dehydrating and other after-nitration treatments may be carried out economically on the sheet nitrocellulose, which is easy to handle and which is a compact and convenient form in which to store and ship nitrocellulose.

In carrying out the process of the present invention, I may use as the first-stage nitrating acid one which is unusually rich in nitric acid but of substantially normal water content. If the acid is maintained at, say, 30 (3., and the sheets are allowed to remain steeped in the acid until an equilibrium nitrogen content is reached, the product will have a nitrogen content short of that desired in the final product, say, about 10%, but the sheets will be substantially uniformly and completely nitrated. One may proceed from this step product, or from one which has not yet reached an equilibrium nitrogen content to a comparatively uniform-final product having a nitrogen content of, say, about 11.5%, and other characteristics desired, by

steeping, by way of example, the sheets constituting the step product in an acid whose composition is or approximates that of the usual nitrating acid.

The initial-stage nitrating acid may be one whose sulphuric acid to nitric acid content ratio is less than about 2 to 1, while that of the second-stage nitrating acid is greater. For instance, this ratio may be, say, 1 to 1, or 1.5 to 1 in the initial-stage nltrating acid and, say, 3 to 1 in the second-stage nitrating acid. After the initial-stage nitration has been effected, the acid used therefor may be drained from the nitrating vat and be re-- placed by the second-stage nitrating acid, The initial stage nitration may be carried on for, say, one-half to two hours, and the final-stage nitration for, say, two hours. The total time consumed is thus reasonably short, so that the nitrating acid and nitrating equipment is not unduly tied up in a single nitration.

In carrying out my process on a commercial scale, the sheets may be supported vertically in closely spaced, parallel relation, as in frames; or they may be suspended as from glass rods or clamps. The sheets should be lowered sufliciently in the vats to permit the baths of nitrating acid to cover them completely, their spaced relation permitting uniform access of acid to all the surfaces of the sheets. Othermodes of carrying out the twostage nitration-of the present invention may, however, be employed without departing from the spirit or scope of invention as defined in the appended claims.

While I find it diificult to account with precision for the results made possible by a two-stage nitration such as described, nevertheless there are certain known factors which probably underlie and are responsible for these results. For instance, it has been observed that the acid used in the first-stage nitration has a comparatively slight gelatinizing or hydrolyzing effect on the sheets un-v der the temperature conditions used. This may be due to the fact that the initial stage nitrating acid is of considerably lower visw cosity than, the usual nitrating acid, and accordingly that it penetrates into the sheets rapidly to produce a high nitric acid concentration throughout the sheets, which permit-s of nitration to an equilibrium nitrogen content without the occurrence of undesirable side reactions. This equilibrium nitro; gen content is lower than that desired in the finished product, by reason of the fact that, other things being equal, with ratios of sulphuric acid to nitric acid less than, say, 3 to 1, lower degrees of nitrogen in the nitrocelluloses results than if higher ratios are used. Hence, after the initialstage nitrating operation, the porous sheets are filled with an acid mixture which is in equilibrium with a comparatively low nitrogen content nitrocellulose. where the nitrating acid is comparatively rich in sulphuric acid, the residual acid in the pores of the sheets resumes reaction gradually and smoothly as it gradually receives sulphuric acid by diffusion from the pool of nitrating acid used in the second stage. In other words, the second stage nitrating acid mingles in the pores of the sheets with the residual acid from the previous treatment, which is sufiiciently strong to prevent side reactions, so that the nitration of the sheets proceeds smoothly to completion.

In some cases, it may be desirable to treat the sheets for a comparatively short period of time with a nitrating acid of a composition corresponding to that used in the secondstage nitration, before the first-stage nitration is effected. This initial treatment is of suificiently short duration so that before the acid within the superficial pores of the sheets reaches the hydrolyzing stage, it is drained off and the nitric-acid-rich, more fluid, firststage nitrating acid is added. Such an initial treatment, which may be, in the nature of a five-minute dip, is of advantage, in that it makespossible the use of a wide variety of acid compositions in the first nitrating stage, without running into the danger of producing 'a final product tending to give hazy nitrocellulose solutions. The choice of a first-stage nitrating acid of the proper composition does not give rise to the foregoing trouble, but there are some acid compositions that do. The nitration of the outer layers of the sheets to completion before the first-stage nitration ensures the production of nitrocelluloses which give clear nitrocellulose solutions.

I shall now give typical procedures falling within the purview ofthe present invention. In these examples, a refined wood pulp of high alpha cellulose content was employed as a.raw material. The pulp was in an unbeaten condition and in the form of drier sheets 0.040 inches thick and weighing 450 grams per square meter. In one case, the sheets were submerged in' fifty times their weight of a nitrating acid at 40 C., of the following composition Water 18.7% HNO 40.6% H SO 40.7%

After two hours, the acid bath was drained from the sheets and a second nitrating acid at After another two hours, the second nitrating acid was drained from the sheets, whereupon the sheets were weighed for acid retention, then weredrowned in a bath of water and after several rinses were placed in boiling In the final stage of nitration,

ill

water for two hours, where they were stabilized. They were thendried and found to be in an integral, firm condition; It was 'found that they had increased 53% in weight,

of the .following composition was used: Water 17.0% HNO 1 33.2%

' 'H so. 49.8%

curves represent the yield of nitrocellulose in In the case of the three experiments,

Other conditions of the previous case were maintained, but both acid baths were. kept at 30 C. In this case, the yield of dry nitrocellulose obtained was 1.56 times the weight of the original dry cellulose, and the average nitrogen content was 11.4%. The acid retention was 1.38 pounds of acid per pound of nitrocellulose. Solutions of the acidified and stabilized nitrocellulose in acetone and in amylace tate were clear and homogeneous. It is to be noted that in.this case-the ratio of sulphuric acid to nitric'acid in the first-stage nitrating acid was 1.5 to 1, rather than l'to 1,

as in the case previously given. The foregoing examples are typical of, a wide variety of times, temperatures, and compositions of nitrating acid which may be used in a process .suchas described, but in any case the principle of using a nitratingvacid rich in nitric acid in the first stage,,and a nitrating acid containinga high ratio of sulphuric acid to nitric acid in the second stage to com plete nitration, is applied.

A In order to arrive at anitrocelluloseof a given nitrogen content, it is well known in the art that nitrating acid of a definite nitric acid to sulphuric acid ratio should have a predetermined water content. Asthe ratio of one acid to the other varies, the water content necessary to produce a nitrocellulose of the same nitrogen-content may vary within relatively narrow limits. Generally speaking, the amount of water present in nitrating acids may be from,, say, 12% to 23% depending uponthe nitrogen content and other characteristics desired in the nitrocellulose, but it is to be understood that the range of water content may also vary, depending upon many factors, including ratio of one acid to the other, the nature of the cellulose, the temperature of nitration, the ratio ofnitrating acid'to cellulose in the bath, etc.

.In order to show the progress of the reaction in a two-stage nitrating process such as described in the examples cited, the curves plotted on the accompanying drawing are submitted. The ordinates of the lower three curves represent the per. cent. nitrogen in the product, and the abscissae represent the time of immersion in hours in the first-stage nitrating acid. The ordinates of the upper two per cent, the uppermost of these two. curves being for two experiments.- The nitrogen and yield curves marked Experiment A were plotted from the results obtained by steeping drier sheets of the kind previously described in a nitrating acid at 40 C.,'of the following composition:

Water -1 18.7% I HNO 40.6% H2804 Water 18.7% HNo. 30.6% H.so. v 50.7%

Experiment C was carried out under the same conditions as Experiment B, except that the second-stage nitrating acid was maintained at 20 C. I present herein the tabulated values of the three experiments from which the curves were drawn:

Experiment A g i Yield 5382:; Appearance in acetone 34 hr. 114. 5 4. 25 Large amount insoluble 1 hr. 132. 2 8. 63 Mglderate amount insoluy e 1% hrs. 139.0 9. 70 Small amount .nsoluble 2 hrs. 142. 0 10.10 Small amount .nsoluble 2% hrs. 142. 3 9. 97 Small amount .nsoluble 3 hrs. 143. 8 9. 87 Solution turbid 3% hrs. 139. 0 9. 73 Solution turbid 4 hrs. 138. 0 9. 92 Solution very turbid Experiment B appear iii: in Time in Time in Percent Yield ancein amyl ace- No. 1 acid No. 2 acid nitrogen aoetone tam 801w tion 2 hr. 3 4m. 146. 2 1o. 29 Clear 1 hr. 3 hrs. 151. 2 11.15 Clear Clear 1% hrs. 2 hrs. 151. 5 11. 18 Clear Clear 2 hrs. 2 hrs. 151. 9 11. 28 1 Clear 2% hrs. 1% hrs. 150. 2 11. 27 Clear 3 hrs. 1 hr. 150. 0 10. 78 Clear 3 hrs. 4 hr ,147. 5 1o. 72 Clear Experiment 0' i A Appear- Time in Time in Percent m Yield ance in amyl ace: No. 1 acid No. 2ac1d nitrogen acetone ate 501m tion hr. 3% hrs. 150. 0 10. a Clear Clear 1 hr. 3 hrs. 150. l 10. 68, Clear Clear 1V hrs 2% hrs. 152. 2 11. 08 Clear Clear 2 hrs 2 hrs 148. 6 10.72 Clear Clear 2% hrs 1% hrs 148. 2 10. 74 Clear 3 hrs. 1 hr. 148. 0 10. 69 Clear 3% hrs. ,6 hr 146. 9 10. 46 Clear 141. 4 9. 80 Clear the - poses. From the plotted results, it is apparthe step product of the first nitration, which H ent that the second-stage nitration increases the yield and actually takes part in the nitration. It is further apparent that the nitrogen content of the nitrocellulose produced may be varied by varying the durations of the treatments in the first-stage and second-/ stage nitrating acids, but in ordinary practice, if it is desired to produce a finished nitrocellulose of about 11.5% nitrogen content,

has a nitrogen content of less than about 11.5%, is nitrated in the second-stage acid for a suflicient period of time toconvert the step product into a finished product of the nitrogen content desired.

The economic advantages of the process herein described are of high significance.

' Not only are yields of nitrocellulose obtained from comparatively thick sheets of unbeaten wood pulp comparable to those obtained when cotton linters is used as the raw material, but the acidlosses are very low.v Since the nitrating temperatures are comparatively high, say, 30 to 40 (1, or even higher, a wide range of viscosities of nitrocellulose is made possible. Because of the comparatively compact nature of the sheets. it is possible to get a comparatively high ratio of cellulose to nitrating acid in the nitrating vats,much

higher. if desired, than when tissue, linters, or

shredded pulp is used, so that, aside from the fact that the raw material is handled in its cheapest form. the nitrating equipment is of the simplest kind, and the labor cost of the process is relatively small.

I claim: 1. A process which comprises the nitration of thick sheets of unbeaten wood pul at temperatures as high as 30 to 40 0., substantially without side reactions.

2. 'A process which comprises the nitration of sheets of substantially unbeaten wood pulp at temperatures as high as 30 to 40 C., while maintaining theintegrity of the sheets.

3. A process which comprises steeping cellulose in a nitrating acid in which the ratio of sulphuric acid to nitric acid is less than 2 to 1, and then steeping the cellulose in a nitrating acid in which the ratio is greater.

4. A process which comprises steeping thick sheets of unbeaten wood pulp in a nitrating acid in which the ratio of sulphuric acid" to nitric acid is less than 2 to 1, and then steeping the sheets while Wet with such acid in anlulose with a nitrating acid in which the ratio of sulphuric acid to nitric acid is between 2 to 1 and 1 to 1,, and then with a nitrating acld 1n which the ratio is greater than in the first nitrating acid and greater than 1 to 1.

6. A process which comprises treating cellulose until artially nitrated with a nitrating acid su ciently rich in nitric acid to avoid material loss throu h side reaction, and then treating the partial y nitrated cellulose with a nitrating acid richer in sulphuric acid content. 7 7. A process which comprises nitrating only the surface of thick sheets of cellulose with one acid, nitrating the bulk of the remainder of the sheets with a second acid, and then nitrating with an acid similar to the rst.

8. A process which comprises immersing thick sheets of cellulose in a bath of nitrating acid to nitrateonly the surface of such sheets, then immersing such sheets in a bath of nitrating acid richer in nitric acid, and then in a bath similar in composition to that used in the first bath.

9. A process which comprises treating thick sheets of cellulose with a nitrating acid in which the ratio of sulphuric acid to nitric acid is greater than 1 to 1 to nitrate only the surface of the sheets, then with an acid richer in nitric acid to. nitrate the rest of such sheets, and finally with an acid in which the ratio is greater than 1 to 1 to increase the nitrogen content of said rest of the sheets.

10. A process which comprises steeping cellulose in a succession of baths of nitrating acid, both the nitric acid content and the nitric acid to s'ulphuricacid ratio of one bath being markedly higher than that of a succeeding bath, and effecting a step-by-step nitration of said cellulose from the first bath' l1. A pr'ocess which comprises steeping a P woodfiber of high alpha cellulose content in a plurality of baths of nitrating acid of different compositions, both the nitric acid content and the nitric acid to sulphuric acid ratioof one bath being markedly higher than that of a succeeding bath, and effecting a step-by-step nitration of said 'wood fiber from the first bath into which it is introduced to the last bath from which the desired nitrocellulose is removed.

12. A process which comprises steeping thick sheets of cellulose in a plurality of baths of nitrating acid of different compositions,

both the nitric acid content and'the nitric acid 'to sulphuric acid ratio of one bath being markedly higher than that of a succeeding removed.

In testimony whereof I have aflixed my signature.

MILTON 0. SCHUR. 

